Often multiple wind turbines (WT) are arranged together and connected in wind power plants (WPP) with a common power plant controller (PPC) for monitoring and controlling the operation of each or at least a number of the wind turbines. In general when operating a wind turbine or a wind power plant it is aimed to obtain maximum yield of the capital invested in it. In order to comply with this, wind turbine control systems are configured to maximise the power output. However, the wind power plant further needs to meet the power production demand and requirement from the utility grid.
A wind power plant may receive a power curtailment command from the grid requiring the power output from the wind power plant to be reduced. The power plant may need to be curtailed due to for example faults on transmission lines, transformer faults, lack of transmission lines capacity, frequency control, power trading, etc. Within certain limits a power curtailment demand from the grid can be handled by power derating one or more of the wind turbines, i.e. reducing the power output of one or more of the wind turbines. However, many wind turbines (especially pitched turbines) cannot be controlled in the whole range of the turbine rated power between zero power to nominal power, but can only be controlled between a technical minimum set point and up to nominal power. Therefore, a curtailment command from the grid may require one or more turbines to be stopped and deactivated to achieve a power output level corresponding to the grid demand.
WO 2013/000473 discloses the controlling of the wind turbines in a wind power plant during curtailment and how to decide which wind turbine is to be deactivated based on different criteria such as for example fatigue data, earlier production compared to potential power production, and other performance criteria.
In general, however, it is not desirable to have a turbine stopped for a very long period without moving or producing power among other reasons because of the need to maintain a minimal lubrication between the machine parts. Traditionally, all stopped or deactivated wind turbines in a power plant are therefore turned on manually from time to time and allowed to produce a limited amount of power irrespective of any curtailment commands from the grid.